Upgrade my iphone 3g to iOS 4. ?

Can i do update my iphone 3g to iOS 4.0

4.1 is the current version. 4.0 is no longer available.

if i do then what things happen to my current phone

It will have the iPhone 3G compatible new features listed here:

http://support.apple.com/kb/DL1061

hi,
I would not upgrade a 3g phone unless it is not functioning. The gains do not outweigh the sacrifice in performance. You have to turn off many features and services to just get the phone to behave close to normal. The 3g lacks the system RAM needed to ket iOS 4 run.

I had no problems on my 3G with 4.1.

If you do plan on upgrading your ios to 4.1, then you just need to plug your iphone to your computer, open itunes, back up the iphone, and install 4.1. Nothing on your iphone will happen unless something goes wrong during the update. Thus, back up before you start upgrading.

When I try to load iOS4.1 it gets hung up backing up the phone. This happens even though I have backed it up before trying to upgrade the software. Any ideas? (I have an iPhone 3G)

What operating system does your computer use?